Validating Strings as Alphanumeric Using Regex in PHP

2025-04-09 2025-04-09

Tak@

To validate alphanumeric characters using regular expressions, use ^[a-zA-Z0-9]+$. This regular expression matches strings composed of one or more alphanumeric characters, such as "3DModel".

If you want to specify a fixed length of alphanumeric characters, use ^[a-zA-Z0-9]{n}$. For example, to match exactly 6 characters, use ^[a-zA-Z0-9]{6}$. Similarly, you can create flexible patterns by specifying minimum and maximum lengths. For instance, ^[a-zA-Z0-9]{12,20}$ is effective for matching strings between 12 and 20 characters.

Below is a summary of commonly used regular expression patterns. Feel free to use them as needed.

#	Match Condition	Regular Expression Pattern
1	Contains only alphanumeric characters	^[a-zA-Z0-9]+$
2	Fixed length of n alphanumeric characters	^[a-zA-Z0-9]{n}$
3	At least n alphanumeric characters	^[a-zA-Z0-9]{n,}$
4	No more than m alphanumeric characters	^[a-zA-Z0-9]{1,m}$
5	Between n and m alphanumeric characters	^[a-zA-Z0-9]{n,m}$

A list of common regular expression patterns for validating alphanumeric

Source Code

Below is a PHP function to validate if a given string consists only of alphanumeric characters. The function allows validation based on the following conditions:

If the minimum length is omitted: Validates that the string is within the specified maximum length and consists only of alphanumeric characters.
If the maximum length is omitted: Validates that the string meets the minimum length requirement and consists only of alphanumeric characters.
If both minimum and maximum lengths are omitted: Validates that the string entirely consists of alphanumeric characters.

/**
 * Checks whether a given string consists only of alphanumeric characters.
 *
 * @param string $str The input string.
 * @param ?int $minLength The minimum number of characters (treated as 1 if null).
 * @param ?int $maxLength The maximum number of characters (treated as the length of the string if null).
 * @return bool Returns true if the input string meets the conditions; otherwise, returns false.
 * @throws InvalidArgumentException Throws an exception if the minimum length is less than 1,
 * or if the maximum length is less than the minimum length.
 */
function isAlphanumeric(string $str, ?int $minLength = null, ?int $maxLength = null): bool {
    // Set default values
    $min = $minLength ?? 1;
    $max = $maxLength ?? mb_strlen($str);

    // Validate arguments
    if ($min < 1) {
        // Throw an exception if the minimum length is less than 1
        throw new InvalidArgumentException('The minimum length must be an integer greater than or equal to 1.');
    }

    if (!is_null($maxLength) && $max < $min) {
        // Throw an exception if the maximum length is less than the minimum length
        throw new InvalidArgumentException('The minimum length must not exceed the maximum length.');
    }

    // Construct the regular expression pattern for alphanumeric characters
    $pattern = is_null($maxLength)
        ? sprintf('/^[a-zA-Z0-9]{%d,}$/', (int)$min) // No limit on the maximum number of characters
        : sprintf('/^[a-zA-Z0-9]{%d,%d}$/', (int)$min, (int)$max);

    // Perform the validation using regular expressions
    return (bool)preg_match($pattern, $str);
}